The cell surface of mammalian cells is in a constant state of flux due to the continuous ingestion of segments of the plasma membrane through the process of pinocytosis. Recent evidence indicates these segments must be conserved and recycled. Two basic approaches will be taken to probe the processing of pinocytic vesicles in fibroblasts. CHO cells will be pulse loaded with pinocytic content markers and the intracellular transport and cycling of marker quantitated. Preliminary experiments indicate horseradish peroxidase is an effective fluid phase pinocytic marker in pulse periods as short as two and one half minutes and that content cycling does occur. Pinocytic membrane will be selectively labeled by lactoperoxidase mediated radioiodination experiments will be directed towards the route of intercellular transport of radioiodinated pinosome membrane proteins and whether this route of appearance at the cell surface is the same as secretory proteins. Work during the past year has established specific conditions for radioiodination and have resulted in data indicating rapid cycling of endocytic membrane proteins to the cell surface. Knowledge of how the cell surface turns over should be important in being able to modulate cell interactions and growth. The ability to label membrane proteins included in pinocytic membrane and to follow these proteins will provide a foundation for future projects in cancer research and aging research.